What is an Entity Identifier?

The identifier of an entity is some set of attributes whose combined value is unique for all instances of that entity.

Thus an entity's identifier is one of its (possibly several) candidate keys.

If an entity has more than one candidate key, the choice of one to be the identifier is an arbitrary convenience for RDBMS operation.

While an entity identifier is not absolutely mandatory, it is hard to think of a useful entity without one.